1. Field of the Invention
The present invention relates to a constraining green sheet and a method of manufacturing a multi-layer ceramic substrate using the same, and more particularly, to a constraining green sheet and a method of manufacturing a multi-layer ceramic substrate using the same, which can improve the firing characteristics by using inorganic powders that are different in terms of density and particle diameters.
2. Description of the Related Art
In general, a multi-layer ceramic substrate using glass-ceramic can provide implementation of a three-dimensional interlayer circuit and formation of cavities. Therefore, with high design flexibility, a multifunctional device can be installed in the multi-layer ceramic substrate.
Due to this, the practical use of multi-layer ceramic substrates is increasing gradually in the market of small-sized and high-functional high-frequency components. The early multi-layer ceramic substrate is manufactured by forming internal circuit patterns and via structures on a ceramic green sheet using conductive paste, laminating it to a desired thickness, and firing the resulting structure. In this process, because the multi-layer ceramic substrate undergoes a volume shrinkage of about 35% to about 50% and a lateral shrinkage is difficult to control, a dimensional error of about 0.5% occurs even in the same manufacturing stage as well as in the respective manufacturing stages.
A design margin of the internal circuit patterns and the via structures decreases with an increase in the structural complexity and fineness of the multi-layer ceramic substrate, thus requiring a non-shrinkage firing process that suppresses a lateral shrinkage of the multi-layer ceramic substrate.
To this end, a method is being widely used that suppresses a planar shrinkage by joining a flexible green sheet of poor-sinterable material, which is not fired at the firing temperature of the ceramic substrate material, onto one side or both sides of the multi-layer ceramic substrate. In particular, a load is applied to prevent a warpage of the ceramic substrate in a firing process. In this case, the firing characteristics may be degraded because a passage for a de-binder of organic materials is not secured in the firing process. Also, there may be a high residual carbon content in the sintered ceramic substrate, thus degrading the reliability of the ceramic substrate.
As a related art de-binder method, the Japanese Patent Laid-Open No. Hei 7-30253 discloses a method that can easily perform a de-binder process by perforating a constraining green sheet and filling the resulting hole with a resin, which is thermally decomposed more easily than an organic binder contained in a ceramic substrate, so that the de-binder operation of the ceramic substrate can be generated sufficiently even when the constraining green substrate is used. This method, however, is problematic in that it must additionally perforate the constraining layer and may cause a device deformation due to the resulting hole.
Also, the Korean Patent Laid-Open No. 2002-0090296 discloses a method that, by using an organic binder, which is lower in thermal decomposition initiation temperature than an organic binder of a device green sheet, for a constraining green sheet, first removes a binder of the constraining green sheet and easily discharges a binder of a device green layer through the resulting passage. However, in order to maximize the constraining force of the constraining green sheet, the contact point between the constraining layer and the ceramic laminated structure must be maximized by fining the powder of the constraining layer and increasing the content thereof. In this case, a blow hole in the constraining green sheet may fail to be sufficiently secured. If the blow hole fails to be sufficiently secured, even though the organic material of the constraining green sheet is first decomposed, it is difficult that a binder burned or decomposed from the ceramic laminated structure is discharged through the blow hole in the constraining green sheet by moving to a thickness of hundreds of microns, thus making it difficult to provide sufficient effects.
Also, in the Japanese Patent Laid-Open No. 2006-173456, as illustrated in FIG. 1, the volume content of an inorganic powder particle 12 and an organic binder 14 of a constraining green sheet 15 is larger in a close surface region 15a to a non-sintered multi-layer ceramic substrate 11 than in a free surface region 15b. That is, the joining force between the ceramic substrate and the constraining layer is increased so that a gradient of the organic material content is generated between the close surface and the free surface, and also a de-binder to the free surface with many blow holes is facilitated.
However, because a doctor blade process is used to form a component density gradient through precipitation in the constraining green sheet 15, it is difficult to guarantee the reproducibility of the volume content and the proper thicknesses of the respective regions. Also, the above method uses an inorganic powder with a large particle diameter (e.g., two or more times than the particle of the ceramic substrate) in order to precipitate the powder particles in the formation of the constraining green sheet and thus reduce the organic binder amount in the bottom thereof. Therefore, it is difficult to sufficiently secure the contact point with the ceramic substrate, and alto it is difficult to increase the capillary force that can move the organic binder from the ceramic substrate to the constraining green sheet.